Interpretive Summary: Nitrogen (N) and phosphorus (P) availability are key elements that regulate plant growth throughout the terrestrial biosphere, influencing the patterns and magnitude of growth by land plants both now and into the future. These nutrients enter ecosystems from external sources (weathering of rocks, atmospheric deposition, N fixation mediated by microbes) and are also recycled internally to varying degrees through the plant-soil-microbe system via organic matter decay processes. However, the proportion of global terrestrial plant growth supported by new nutrient inputs versus recycled nutrients is unresolved, as are the large-scale patterns of variation across terrestrial ecosystems. Here, we combined satellite imagery, ecosystem modeling and empirical observations to identify previously unrecognized patterns of new versus recycled nutrient (N and P) plant growth on land. Our analysis points to tropical forests as a hotspot of new growth fueled by new N (accounting for 45% of total new NPP globally), much higher than previous estimates from temperate and high latitude regions. The large fraction of tropical forest growth resulting from new N is driven by the high capacity for N fixation, though this varies considerably within this diverse biome; N deposition explains a much smaller proportion of new growth. By contrast, the contribution of new N to growth is lower outside the tropics, and worldwide, new P inputs are uniformly low relative to plant demands. These results imply that new N inputs have the greatest capacity to fuel additional growth by terrestrial plants, whereas low P availability may ultimately constrain growth across much of the terrestrial biosphere.

Technical Abstract:
Nitrogen (N) and phosphorus (P) availability regulate plant productivity throughout the terrestrial biosphere, influencing the patterns and magnitude of net primary production (NPP) by land plants both now and into the future. These nutrients enter ecosystems via geologic and atmospheric pathways, and are recycled to varying degrees through the plant-soil-microbe system via organic matter decay processes. However, the proportion of global NPP that can be attributed to new nutrient inputs versus recycled nutrients is unresolved, as are the large-scale patterns of variation across terrestrial ecosystems. Here, we combined satellite imagery, biogeochemical modeling and empirical observations to identify previously unrecognized patterns of new versus recycled nutrient (N and P) productivity on land. Our analysis points to tropical forests as a hotspot of new NPP fueled by new N (accounting for 45% of total new NPP globally), much higher than previous estimates from temperate and high latitude regions. The large fraction of tropical forest NPP resulting from new N is driven by the high capacity for N fixation, though this varies considerably within this diverse biome; N deposition explains a much smaller proportion of new NPP. By contrast, the contribution of new N to primary productivity is lower outside the tropics, and worldwide, new P inputs are uniformly low relative to plant demands. These results imply that new N inputs have the greatest capacity to fuel additional NPP by terrestrial plants, whereas low P availability may ultimately constrain NPP across much of the terrestrial biosphere.